System and method for pH control of lean MEG product from MEG regeneration and reclamation packages

ABSTRACT

A lean MEG stream having a first pH level is contacted with a CO2-rich gas stream to yield a lean MEG product having a second different and lower pH level preferably in a range of 6.5 to 7.0. The system and method can be readily incorporated into a slipstream MEG recovery package, with a source of the lean MEG stream being a MEG regeneration section of the package. The CO 2 -rich gas could be a vented CO 2  stream from the MEG reclamation section of the package. Unlike hydrochloric and acetic acid overdosing, CO 2  overdosing of the lean MEG stream does not lead to rapid acidification of the lean MEG product to be stored or injected.

BACKGROUND OF THE INVENTION

Slipstream MEG recovery packages use a regeneration section to removewater from an incoming rich MEG feed stream and produce a lean MEGstream. A portion of this lean MEG stream is routed to a reclamationunit or section where the salt component is removed to yield asalt-free, pH neutral, lean MEG stream. This salt-free lean MEG streamis then blended with the remaining lean MEG stream to produce a lean MEGproduct having up to 3 wt % dissolved salts and available forre-injection into the gas production line as hydrate inhibitor.

For gas fields where significant quantities of calcium and otherdivalent cations are present in the formation water, a calcium removalunit or section is located upstream of the regeneration section. Thecalcium is removed from the rich MEG stream by elevating the pH throughthe addition of sodium or potassium carbonates, hydroxides, or somecombination thereof. The lean MEG exits the calcium removal section withan elevated pH, typically above 9.5.

Because carbonate and hydroxide are often added in excess of therequired stoichiometric quantity, un-reacted carbonate and hydroxide iscarried through the regeneration system and into the lean MEG product.Removal of water from the rich MEG in the regeneration section furtherelevates the pH of the lean MEG product sent for re injection to above10. Mixing this high pH lean MEG with the calcium-rich formation waterin the gas production pipeline can lead to increased scaling of thepipeline by precipitation of, for example, calcium carbonate.

Therefore, a need exists to reduce the pH of the lean MEG product priorto injection and, in turn, mitigate pipeline scaling. Acidification ofthe lean MEG with hydrochloric acid (HCl) is an option but overdosingwith hydrochloric acid can lead to rapid reduction in pH to levels atwhich corrosion of carbon steel pipework and vessels may occur.

SUMMARY OF THE INVENTION

A lean MEG stream having a first pH level (e.g., pH>9.5) is contactedwith a CO₂-rich gas stream to yield a lean MEG product having a seconddifferent pH level preferably in a range of 6.5 to 7.0. The CO₂-rich gascould be a vented CO₂ stream from a MEG reclamation unit.

Carbon dioxide is preferred to hydrochloric acid (HCl) and acetic acid(CH₃CO₂H) for pH control because overdosing with CO₂—i.e., adding it inexcess of the required stoichiometric quantity—does not lead to thesignificant reduction in pH observed with hydrochloric acid or theaccumulation of acetates observed with acetic acid.

Objectives of this invention include providing a system and method thatreduces the pH of lean MEG product prior to injection, mitigates thepotential for pipeline scaling, does not make use of dosing with organicor inorganic acids to control the pH of the lean MEG product, is lesssensitive to overdosing conditions than those organic and inorganicacids, and does not cause rapid reduction in pH levels when an overdosecondition occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a preferred embodiment of a system and methodof this invention. A vessel located downstream of a MEG regenerationsection receives a high pH lean MEG stream and allows the steam to comeinto contact with a CO₂-rich gas.

FIG. 2 is a graph illustrating a lean MEG stream with alkalinity presentas sodium carbonate as the stream is treated with CO₂, acetic acid, andhydrochloric acid.

FIG. 3 is a graph illustrating a lean MEG stream with alkalinity presentas sodium hydroxide as the stream is treated with CO₂, acetic acid, andhydrochloric acid.

ELEMENTS AND NUMBERING USED IN THE DRAWINGS

-   -   10 Vessel    -   15 Rich MEG steam    -   20 Lean MEG stream    -   21 Portion of lean MEG stream 20    -   30 MEG regeneration unit or section    -   40 CO₂-rich gas 40    -   50 Lean MEG product exiting 10    -   60 MEG reclamation unit or section    -   61 Salt-free lean MEG stream    -   70 Calcium removal unit or section

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a preferred embodiment of a system and method foradjusting a pH level of a lean MEG steam includes a vessel 10 whichreceives a lean MEG stream 20 from a lean MEG source such as aregeneration unit or section 30 of a slipstream MEG recovery package.Typically, stream 20 has a pH level above 9.5, as does rich MEG stream15 upstream of the regeneration section 30. Within vessel 10, this highpH lean MEG stream 20 comes into contact with a CO₂-rich gas 40 (i.e.,greater than 50% CO₂ content). Vessel 10 can be a contactor vessel of akind known in the art.

The CO₂ in gas 40 forms acidic solutions when dissolved in the MEG-watermixture of stream 20, thereby reducing the pH. A lean MEG product 50having a second lower pH exits the vessel 10. Preferably, product 50 hasa pH level in a range of 6.5 to 7. No inorganic acids such as HCl ororganic acids such as acetic or citric acid is required for reducing thepH to this level.

The CO₂-rich gas 40 can be from any source preferable but is morepreferably a vent stream from a reclamation unit or section 60 of theslipstream MEG recovery package. Similar to MEG regeneration section 30,MEG reclamation section 60 is of a kind well-known in the art.

A salt-free lean MEG stream 61 which exits the reclamation section 60can be mixed with the lean MEG stream 20 prior to stream 20 enteringvessel 10. Additionally, a portion 21 of the lean MEG stream 20 whichexits the regeneration section 30 can be routed to the reclamation unit60.

In slipstream MEG recovery packages that make use of a calcium removalunit or section 70 upstream of the regeneration unit 30, excesscarbonate that finds its way into the reclamation section 60 degrades toform CO₂ (and hydroxide) under the elevated temperature, low pressureregime of a flash separator (not shown).

Referring to FIGS. 2 and 3, unlike hydrochloric and acetic acidoverdosing, CO₂ overdosing within vessel 10 does not lead to rapidacidification of the lean MEG product 50. In a CO₂ overdosing condition,the pH level remains above 6 whereas in an acetic acid and hydrochloricacid overdosing condition the pH level falls below 4 and 2 respectively.Therefore, the system and method of this invention is less sensitive tooverdosing conditions than prior art methods.

As mentioned above, acidification with CO₂ removes the risk which occurswith inorganic acids (HCl) and the absence of carboxylates (acetate),namely, overdosing to the point of potentially damaging pH levels. Inaddition, carboxylates are highly soluble in MEG and are difficult toremove once added to the MEG system. The accumulation of carboxylatescan lead to operational problems as the density and viscosity of the MEGincreases with increasing carboxylate content. Hydrochloric acidconverts readily to salt plus water; carbon dioxide converts tobicarbonate which is much more easily managed in the MEG system thancarboxylates. Although the CO₂ reduces the pH, the ‘alkalinity’ (OH—plus HCO₃— plus CO₂) is not reduced.

To examine the “scaling” potential for the system and method, thefollowing software simulation was run employing OLI Analyzer v 9.1.5(OLI Systems, Inc., Cedar Knolls, N.J.).

Starting Solution:

90 wt % MEG (on salt-free basis) at 40° C. containing 30,000mg/kg_(solvent) sodium chloride, 250 mg/kg_(solvent) of sodium carbonateand 25 mg/kg_(solvent) of sodium hydroxide. The pH of this mixture was10.053 or about 10 (see Table 1, col. A, below).

Acidification:

The MEG solution was neutralized to pH=7.0 and to pH=6.5 using HClacetic acid and CO₂. Quantities of HCl, CH₃CO₂H and CO₂ added are shownin Table 1, rows 12-14, below.

Scaling Test:

Scaling potential of the acidified solutions was determined by adding inseparate simulations MgCl₂, CaCl₂, FeCl₂, SrCl₂ and BaCl₂ to the leanMEG solutions at the quantities shown in Table 1, rows 19-23.

TABLE 1 Software Simulation of Scaling Potential.  1 A B C D E F G  2  3TEMP 40 40 40 40 40 40 40  4  5 H2O g 100,000 100,000 100,000 100,000100,000 100,000 100,000  6 MEG g 900,000 900,000 900,000 900,000 900,000900,000 900,000  7 NaCl g 30,000 30,000 30,003 30,000 30,000 30,00030,000  8 Na2CO3 g 250 250 250 250 250 250 250  9 NaOH g 25 25 25 25 2525 25 10 11 ACIDIFICATION 12 HCl g 0 136 — — 160 — — 13 CHCO2H g 0 — 228— — 279 — 14 CO2 g 0 — — 239 — — 478 15 16 pH — 10.05 7.01 7.01 7.016.50 6.50 6.50 17 18 SCALING TEST POST ACIDIFICATION 19 MgCl2 for Mgprecipitation as Mg(OH)2 g 20 CaCl2 for Ca precipitation as CaCO3 g 1.4840 880 250 4300 5300 800 21 FeCl2 for Fe precipitation as FeCO3 g 0.11.5 1.7 0.8 6.7 7.7 2.4 22 SrCl2 for Sr precipitation as SrCO3 g 0.5 650660 200 3900 3950 620 23 BaCl2 for Ba precipitation as BaCO3 g 0.2 713.7 2.1 35 80 6.3

Results:

For the starting solution (col. A, pH=10.0) precipitation of divalentcations as carbonate occurs on addition of 1.4 g of CaCl₂. Afteracidification to pH 7.0 with HCl, the quantity of calcium chloride addedbefore precipitation of CaCO₃ increases to 840 g from 1.4 g. The effectwith acetic acid is similar with precipitation starting at 880 g ofCaCl₂. The equivalent scaling point with carbon dioxide occurs at 250 g,less than that for HCl or acetic acid but a considerable improvement onthe 1.4 g for the untreated sample.

Similar trends are observed for the other divalent cations (Fe, Sr, Ba)although some are more insoluble than others. Iron, in particular, tendsto precipitate out readily. At pH=6.5 (col. E-G) the trends agree withthose shown at pH=7.0 (col. B-D), i.e. precipitation of divalent cations(Ca, Fe, Sr, and Ba) from the lean MEG is inhibited by addition of CO₂to the alkaline lean MEG mixture.

While preferred embodiments have been described, the invention isdefined by the following claims and their full range of equivalency.

What is claimed:
 1. A method of adjusting a pH level of a lean MEGstream, the method comprising the step of contacting a lean MEG streamwith a CO₂-rich gas, the lean MEG stream having a first pH level beforecoming into contact with the CO₂-rich gas and, after coming into contactwith the CO₂-rich gas, having a second different pH level.
 2. A methodaccording to claim 1 wherein an amount of the CO₂-rich gas is at leastequal to a stoichiometric quantity effective for achieving a desiredsecond lower pH level.
 3. A method according to claim 1 wherein theamount of CO₂-rich gas exceeds the stoichiometric quantity.
 4. A methodaccording to claim 1 wherein the second different pH level is at least6.
 5. A method according to claim 4 wherein the second different pHlevel is
 7. 6. A method according to claim 1 wherein a source of theCO₂-rich gas is a vented CO₂ stream from a MEG reclamation unit.
 7. Amethod according to claim 1 wherein upstream of the vessel the lean MEGstream is mixed with a second lean MEG stream having a different pHlevel than the first pH level of the lean MEG stream.
 8. A methodaccording to claim 7 wherein a source of the second lean MEG stream is aMEG reclamation unit.
 9. A method according to claim 7 wherein thesecond lean MEG stream has a pH level of
 7. 10. A method according toclaim 1 wherein a source of the lean MEG stream is a MEG regenerationunit.